Coordinated Multi-Point (CoMP) transmission technology has been applied in a Long Term Evolution-Advance (LTE-A) system, to thereby reduce interference from an adjacent cell for a User Equipment (UE) at the edge of a coverage area of a small cell, so as to improve an experience of the UE at the edge of the cell. Coordinated Multi-Point (CoMP) transmission technology refers to coordination between multiple Transmission Points (TPs) separate in geographical position. Typically multiple transmission points refer to base stations of different cells, or a base station of a cell and multiple Remote Radio Heads (RRHs) controlled by the base station. CoMP transmission technology can be categorized into downlink coordinated transmission and uplink joint reception. Downlink coordinated multi-point transmission is generally further categorized into two transmission schemes of Coordinated Scheduling/Coordinated Beam-forming (CS/CB) and Joint Processing (JP). In the CS/CB scheme, one of multiple transmission points transmits a useful signal to the UE, and interference of the other transmission points to the UE is reduced as much as possible through joint scheduling and beam-forming. The joint processing scheme can be further categorized into Joint Transmission (JT) scheme and Dynamic Point Selection (DPS) scheme. In the JT scheme, multiple transmission points transmit useful signals to the UE concurrently, to thereby enhance the received signal of the UE. In the DPS scheme, the transmission point to the UE is switched dynamically, by selecting the optimum one for the UE among the cooperating transmission points, to transmit a signal to the UE. These schemes of coordinated multi-point transmission may be applied in combination with each other, or may be combined with Dynamic Blanking, to disable some transmission points from transmitting signals over some time-frequency resources.
The base stations needs to exchange a large amount of information and data in coordinated multi-point transmission. Information and data are exchanged between the base stations in a Long Term Evolution (LTE) system via an X2 interface. An information transmission rate and a transmission delay via the X2 interface are determined by the characteristic of a physical link, and the delay of a protocol stack, of the X2 interface. If the base stations are connected by a high-capacity physical link, e.g., connected directly by an optic fiber, then there is a high information transmission rate via the X2 interface (e.g., at the order of 1 Gbps). If the base stations are connected by a low-capacity physical link, e.g., a radio transmission link, then there is a low information transmission rate via the X2 interface (e.g., 1 Mbps or lower). The delay via the X2 interface arises primarily from the delay of the physical layer transmission, and the delay of the protocol stack and may be up to 10 ms or more. There may be a variety of physical connection modes of the X2 interface in a practical network, and information shall be exchanged between the base stations in CoMP coordinated scheduling by taking a non-ideal X2 interface into account.
Downlink coordinated multi-point transmission is realized based upon Channel State Information (CSI) obtained by the base stations, and the CSI are information of the channels from the UE to the respective transmission points. The CSI information includes Channel Quality Indicator (CQI) information, Pre-coding Matrix Indicator (PMI) information, Rank Indicator (RI) information, etc. The UE measures information of the channels from the respective base stations to the UE by using downlink reference signals transmitted by the base stations, and feeds the channel state information measured by the UE to a serving cell of the UE. The serving cell of the UE receives the fed-back CSI information and performs coordinated scheduling and/or coordinated pre-coding with the cooperating cells, to thereby enable coordinated transmission. Schemes of coordinated scheduling and/or coordinated pre-coding between the cells can be categorized into centralized coordinated scheduling and distributed coordinated scheduling.
Centralized coordinated scheduling generally includes the following operations:
A. The respective cooperating base stations transmit the received CSI information of all the UEs accessing the respective base stations to a Central Coordination Node (CCN);
B. The CCN centrally schedules time and frequency resources for all the UEs of the cooperating base stations, and calculates pre-coding for the UEs for which the time and frequency resources are scheduled;
C. The CCN transmits scheduling and pre-coding results of the respective UEs to the respective related base stations; and
D. The base stations transmit data for the UEs according to the received scheduling and pre-coding results.
In the centralized coordinated scheduling scheme, the scheduling CCN may perform global optimized scheduling for all the UEs in the cooperation set according to the global CSI information, to thereby achieve an ideal cooperation gain. However as demonstrated in the operations of centralized coordinated scheduling, centralized coordinated scheduling requires the cooperating base stations to transmit the CSI information of all the accessing UEs to the CCN, and the CCN needs to transmit all the scheduling results respectively to the respective base stations after scheduling, as illustrated in FIG. 1. Transmission needs to be performed at least twice between the CCN and the base stations, so that with respect to a non-ideal X2 port connection, there may be a significant delay for the transmission via the X2 port, thus resulting in a considerable scheduling delay and a loss of transmission performance.
A general principle of distributed coordinated scheduling lies in that the respective cooperating base stations schedule respectively; and the cooperating base stations cooperate by exchanging the scheduling information, the CSI information of the scheduled UEs, etc.
From the perspective of the principle, distributed coordinated scheduling is performed respectively at the respective cooperating base stations without exchanging a large amount of CSI information via the X2 interface, so that there is a less amount of information transmitted via the X2 interface than in centralized coordinated scheduling, as illustrated in FIG. 2. However, distributed coordinated scheduling can not optimize a global scheduling result according to scheduling conditions between the base stations. In order to achieve a nearly globally-optimized result, iterative scheduling between the base stations may need to be performed, so that the cooperating base stations may need to exchange the scheduling information with each other repeatedly. If there is a significant delay via the X2 interface between the base stations, then repeated exchanges of the scheduling information in distributed coordinated scheduling may come with such a high scheduling delay that the channel information may become outdated, thus degrading the transmission performance Thus, the scheme of CoMP distributed coordinated scheduling needs to be designed carefully in the scenario with a significant delay via the X2 interface, to thereby minimize the number of times that the information is exchanged between the cooperating base stations, to thereby lower the amount of exchanged information.
In summary, there is such a large amount of information exchanged between the base stations in the existing scheme of CoMP centralized coordinated scheduling that there may be a significant loss of the system performance, if the scheme is implemented by using a non-ideal link of the X2 interface; and the scheduling information needs to be exchanged iteratively in iterative distributed coordinated scheduling, so that there will be a considerable increase in transmission delay via the X2 interface with a significant delay.